1. Field of the Invention
The present invention relates to a negative electrode for a nickel-hydrogen rechargeable battery and a nickel-hydrogen rechargeable battery using the negative electrode.
2. Description of the Related Art
One well-known type of alkaline rechargeable battery is the nickel-hydrogen rechargeable battery. The nickel-hydrogen rechargeable battery has a higher capacity and is environmentally safer than a nickel-cadmium rechargeable battery. For these reasons, the nickel-hydrogen rechargeable battery is used for a variety of applications including various portables, hybrid electric vehicles, etc. Because of these various applications, it has been anticipated that the nickel-hydrogen rechargeable battery will be further increased in capacity.
The capacity of the positive electrode of a nickel-hydrogen rechargeable battery is determined by the amount of nickel hydroxide held in a positive electrode plate. The capacity of the negative electrode of the nickel-hydrogen rechargeable battery is determined by the amount of hydrogen-storage alloy held in a negative electrode plate. Accordingly, in order to increase the capacity of the nickel-hydrogen rechargeable battery, it is necessary to increase the amount of the nickel hydroxide and the hydrogen-storage alloy. However, the size of batteries is standardized, and it is impossible to increase internal volume. In order to increase the battery capacity, it is required to increase the filling density of the nickel hydroxide and the hydrogen-storage alloy to be infiltrated into electrode plates.
In the case of the nickel-hydrogen rechargeable battery, the negative electrode is designed to have higher capacity than the positive electrode because, when the battery is overcharged, the oxygen gas produced at the positive electrode is reduced in the negative electrode to prevent the inner pressure of the rechargeable battery from increasing. It is then highly required that the hydrogen-storage alloy in the negative electrode of the nickel-hydrogen rechargeable battery is highly densified. The negative electrode having high-density hydrogen-storage alloy is generally fabricated as below. First, a negative mixture layer containing hydrogen-storage alloy powder is attached to a core body serving as an electrode plate such as punching metal sheet. In the next step, the negative mixture layer undergoes a rolling process by being applied with a high rolling load. This way, a negative electrode in which hydrogen-storage alloy is highly densified is fabricated (see Unexamined Japanese Patent Publication No. 2000-228201, for example).
If the rolling load is increased to achieve the higher density of the hydrogen-storage alloy in the negative electrode, hydrogen-storage alloy particles are pulverized and decreased in diameter. If the diameter of the hydrogen-storage alloy particles is decreased, this increases the total surface area per unit mass, namely, specific surface area, of the hydrogen-storage alloy powder contained in the negative electrode. If the specific surface area is increased, a region of contact between the hydrogen-storage alloy and alkaline electrolyte grows larger. Accordingly, the corrosion reaction of the hydrogen-storage alloy is accelerated. This causes the problem that the cycle life of the battery is shortened.
One considerable way to prevent such a problem is to utilize hydrogen-storage alloy particles having large diameter. In other words, the hydrogen-storage alloy particles having large particles are not easily pulverized by being applied with a high rolling load, so that the diameter of the particles remains large, and the specific surface area of the hydrogen-storage alloy powder is decreased. As a result, the corrosion reaction of the hydrogen-storage alloy is delayed, and improvement in cycle life characteristics can be expected.
However, if the particle diameter of the whole hydrogen-storage alloy powder is simply increased, this increases damage caused on the core body by the hydrogen-storage alloy particles during the rolling process. The negative electrode produced in this manner has an undulating shape instead of being flat, and is thus deteriorated in shape quality. If this low-quality negative electrode is rolled with a separator and a positive electrode, an electrode group thus produced has a deformed shape. This makes it impossible to assemble a battery.